
#73
Jul1009, 05:30 PM

P: 162

For the solid to have a net torque, the torque at its centre of mass must be nonzero. If 'F' is applied at the centre of mass of the wheel and there is no friction, then the wheel will not rotate. But if 'F' is applied somewhere other than the centre of mass, it will produce a net torque about the centre of mass and set the wheel rotating on its own. When there is a friction force at 'P', it too produces torque at the centre of mass of the wheel. If 'F' is applied at the centre of mass, the it is the friction force at 'P' that is responsible for setting the wheel in rotational motion. At least I think so. I don't understand what you're saying about the movements from different perspectives. Please explain? 



#74
Jul1109, 01:41 AM

P: 692

Please bear with me if I'm being a little stupid, but I've always had an ambiguity with this topic. 



#75
Jul1109, 01:52 AM

P: 288

Static friction does work in certain situations when the surface itself accelerates with respect to the observer. Consider the example of two blocks placed one above the other on a frictionless surface. By pushing the lower block of mass M with a force F, the upper block of mass m, also accelerates because the static friction is doing work on it. (Assuming the value of [tex]\frac{Fm}{M+m}[/tex] is below the max static friction force)
Back to our good ol wheel. The point P(the contact point of wheel with the ground) does come to zero velocity each time before being uplifted by the centripetal force. It cannot have the "uniform velocity" as you mentioned, because anything above zero isn't admissible to the earth's surface. Its just like me. tooo lazy!. Watch the cycloid Urmi. 



#76
Jul1109, 05:51 AM

P: 162

Yes?:P 



#77
Jul1109, 05:55 AM

P: 162





#78
Jul1109, 05:58 AM

P: 288





#79
Jul1109, 06:06 AM

P: 288





#80
Jul1109, 06:19 AM

P: 162

About the net force on 'P' (I'm assuming we are considering the situation that Urmi Roy described: the wheel subject to a tangential force 'F' (applied at its centre of mass?), and a static friction force at the point of contact 'P', thus the wheel has both a linear acceration and an angular acceleration (both of them compatible with the no slipping condition).
So! basically, the centre of mass has a linear acceleration (and no angular acceleration), so the net force upon it is a linear force to the right. And all points, including point 'P', have the same linear acceleration as the centre of mass, plus an angular acceleration about the centre of mass (superposition of two movemets). Thus, they are all subject to the same force that the centre of mass is, plus a centripetal force directed toward the centre of mass equal to the 'mass of the particle' times the distance of the particle to the centre of mass and the square of the angular velocity of the wheel at each instant. 



#81
Jul1109, 06:24 AM

P: 162

but I'm correct in saying that in your example the friction force doesn't do 'it's own' work, just transmits part of the work done by the force 'F'. And its true that friction cannot provoke movement on its own, it cannot transform some other kind of energy into kinetic energy! that's all I'm trying to say. At least this is true? 



#82
Jul1109, 06:33 AM

Mentor
P: 40,889





#83
Jul1109, 07:29 AM

P: 692

Also, please tell me how I could modify my understanding of 'rolling' by stating what is wrong and what is right about my post(post 72 of this thread). 



#84
Jul1109, 09:58 AM

P: 162

That's why we talk of the moment (or torque) with respect to a point. Maybe you're mixing this idea with different referece frames? I really don't think there's any relationship, but if you find there is let me know! 



#85
Jul1209, 11:11 PM

P: 288





#86
Jul1209, 11:44 PM

P: 288





#87
Jul1309, 12:22 AM

P: 692





#88
Jul1309, 01:16 AM

P: 288





#89
Jul1309, 01:17 AM

P: 288





#90
Jul1309, 02:41 AM

P: 692

Actually I don't have anything new. As I said,this issue of 'rolling friction' has been bothering me for quite a while,the main difficulty of which stemmed from a particular extract of a book I read.
So,after getting together all that Doc Al, BobbyBear, vin300 and ofcourse, you said, I just tried to summarise the new picture in my head, in post 72. Since then,you pointed out that viewing it from the aspect of individual particles isn't necessary, so apart from that, I just wanted to confirm my idea (as presented in #72) was basically right. If you think I have any major problems, please modify my post and tell me the true 'picture'. 


Register to reply 
Related Discussions  
rotational mechanics  General Physics  3  
Rotational Mechanics  Introductory Physics Homework  13  
Rotational Mechanics  Introductory Physics Homework  3  
Rotational mechanics  Introductory Physics Homework  4  
Rotational Mechanics  Introductory Physics Homework  9 